First prepare string variable which contains all selected values in listbox separated by comma(,) with following code
string selecteditems = "";
for (int i = 0; i < listBox1.Items.Count; i++)
selecteditems += "'"+listBox1.GetItemText(listBox1.Items[i])+"',";
selecteditems = selecteditems.Remove(selecteditems.Length - 1);
Now write ur select query as
" Select * from clienttable where client_name in ("+ selecteditems +")"
this will surely help u..
hav a great time.
If it helps u dnt forget to click on accept ans and vote for it.